Process for alignment of sheet material on a reference edge

ABSTRACT

The invention relates to a process for alignment of sheet material ( 1 ) which is processed in a machine which processes sheet material ( 1 ). To do this the sheet material ( 1 ) is aligned into one conveyor plane ( 9 ) laterally and with respect to its twisting ( 6 ) with reference to its conveyor direction ( 22 ). The position of the sheet material ( 1 ) in the conveyor plane ( 9 ) is always acquired at a point ( 35 ) on one side edge ( 24 ) of the sheet material with a position which is defined to the reference edge ( 23 ) of the sheet material ( 1 ).

[0001] The invention relates to a process for alignment of sheet material on a reference edge, before the sheet material is processed further in a machine which processes sheet material.

[0002] DE 44 16 564 A1 discloses a sheet alignment device. This device for alignment of a sheet moving along an essentially flat transport path enables alignment of a moving sheet in a plurality of orthogonal directions, for example transversely to the transport path, in the direction of the transport path, and to eliminate skewed positions. The sheet alignment device has a first roller arrangement with a first pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. A second roller arrangement has a second pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. There is a third roller arrangement which has a third pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. The third roller arrangement which can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path can be moved along its axis of rotation in the direction which runs transversely to the transport path. Finally, there is a control means which is dynamically connected to the first and the second and the third roller arrangement and selectively controls the rotation of the first and second roller arrangement in order to align the front edge of a sheet moving in the direction of sheet transport along the transport path into the position which is at a right angle to the direction of sheet transport. The control means furthermore controls the rotation and the transverse motion of the third roller arrangement in order to align the moving sheet in the direction which runs transversely to the direction of sheet transport and in the direction in which the sheet is moving along, the transport path.

[0003] The alignment device known from DE 44 16 564 A1 enables the required alignment accuracies to be satisfied only to a limited degree. To achieve the required alignment accuracies for sheet material, extensive modification of the sheet alignment device of the prior art is necessary, which modification does not seem economical.

[0004] In sheet-processing printing presses which work using the offset principle, the sheets are conveyed on the feed table in a ragged arrangement before they can be aligned on the side and pull-type lay marks which are provided in the plane of the feed table. After completed alignment of the sheet material it is transferred in the aligned state to a pre-gripper which accelerates the sheet material to the machine speed and transfers it to the sheet-guiding cylinder downstream of the pre-gripper means. Other alignment concepts generally use cylindrical rollers with a rubber coating which can be held on their core. If with this configuration alignment of sheet material is carried out during its feed by changing the speed between the left and right roller which grip the sheet material, the sheet material undergoes rotation around a pivot. The latter can be located either on the stationary roller or during feed of the sheet material can be located outside the roller with lower rpm or between the two rollers.

[0005] If the side edge of the sheet is acquired with simple photoelectric barriers, a genuine measured value for the pertinent edge position of the sheet material is not available. Photoelectric barriers can only provide a binary-value information about the deviation of the sheet material, whether it is too far to the left or too far to the right; for lateral alignment the sheet material is then pushed until the photoelectric barrier operates. The time necessary for the lateral alignment of the sheet material depends on the initial magnitude of the side register error. If lateral alignment takes place during motion of the sheet material in the conveyor direction, the point in the sheet conveyor direction at which the photoelectric barrier operates is variable. For a small side register error the alignment is less and the distance between the front edge of the sheet and the operating point on the side edge of the sheet is moreover smaller than for larger side register errors which develop. Thus, in this alignment concept the cutting tolerances of the sheet material, i.e. the right angle between the front edge and the side edge, are included in the alignment. In printing booklets in which A-3 format is printed, accordingly alignment is done with respect to the front edge and side edge, then folding to A 4 is done and then stitching, and in which pictures extend over two sides of different sheets, the problem thus arises that register accuracy on the folded spines can be achieved only within the framework of the cutting tolerance; this is frequently considered inadequate.

[0006] The object of the invention in view of the approach known from the prior art is to keep the alignment accuracy of the sheet material during its feed in the conveyor direction independent of processing tolerances of the sheet material upon which an image is to be printed.

[0007] This object as claimed in the invention is achieved by the features of claim 1.

[0008] The advantages which can be achieved with the approach as claimed in the invention are mainly that for each copy of the sheet material its position can be measured at always the same position with respect to a reference edge and afterwards controlled correction of the sheet position is carried out. Thus the image position on the sheet material is always clearly defined relative to the reference edge, for example the front edge of the sheet and the reference point on the side edge of the sheet material. This results in that the register accuracy is no longer limited by the cutting tolerance of the sheet material in the finishing of the sheet material for example in production and folding.

[0009] According to one advantageous version of the process proposed as claimed in the invention, line sensors can be used to advantage to determine the position of the sheet material in the conveyor plane. The line sensors can advantageously be made as CCD lines or also as diode lines. By using line sensors, on the one hand a plurality of commonly processed formats of sheet material can be acquired in their lateral position; furthermore, by using line sensors there is high resolution for accurate acquisition of the lateral position of the sheet material.

[0010] With the process as claimed in the invention, the sheet material can be conveyed further in the conveyor direction especially during its alignment. This ensures that the feed rate of the sheet material to a downstream processing machine is not limited by alignment and its speed.

[0011] With the process as claimed in the invention the alignment accuracy becomes independent of the processing tolerances, such as for example the cutting tolerances of the sheet material, whether paper, cardboard, or even film, since the position of the sheet material is always acquired at the same location, formed from the reference point on one edge and its defined position to a reference edge of the sheet material.

[0012] With reference to the always-identical acquisition of the position of the sheet material in the conveyor plane, the position of the printed image on the sheet material is always defined over reference distances. The first reference distance is formed by the distance of the printed image from the reference edge of the sheet material. Preferably the reference edge of the sheet material is the front edge of the sheet. The second reference distance which determines the position of the printed image relative to the sheet material is the distance of the printed image from the reference point, for example the reference point on the side edge of the sheet material at a short distance in the conveyor direction of the sheet material horizontally from the front edge.

[0013] In a preferred application the process proposed as claimed in the invention can be carried out to improve the alignment accuracy on alignment units which can be connected upstream of the printing group of a digitally operating rotary press. There the print material to be processed, whether film, cardboard or paper, is aligned before it is printed or optionally is processed in a digitally operating machine.

[0014] The invention is detailed below using drawings.

[0015]FIG. 1 shows the developing position deviation of a printed image relative to the surface of the print material which holds it,

[0016]FIG. 2 shows the offset of the printed image on the sheet material, i.e. the offset characterized by a rotary offset,

[0017]FIG. 3 shows the offset which has been printed on the bottom and top of sheet material in perfecting,

[0018]FIG. 4 schematically shows a side view of the sheet feed area of a sheet processing machine,

[0019]FIG. 5 shows a plan view of the alignment components, sensor technology and drives for the sheet material relative to the rotation elements which align the direction in which the sheets run,

[0020]FIG. 6 shows the rotation element which is made as segmented rollers above the conveyor plane of the sheet material for alignment,

[0021]FIG. 7 shows the alignment of sheet material with the drives of the segmented rollers which carry out alignment,

[0022]FIG. 8 shows the effect of tolerances of the printed material on the lateral alignment and

[0023]FIG. 9 shows the definition of the reference edges and the reference point on the sheet material.

[0024]FIG. 1 shows sheet material, for example a printed sheet 1, which is oriented at a right angle to its feed direction. The printed sheet 1 contains on its surface a printed image 2 which is surrounded by a frame-like edge 3. The deviations of Ax and Ay which are marked within the printed surface 2 and the frame 3, designating the positioning errors in the x and y direction 4 and 5, can be adjusted when printing the image 2 onto the surface of the sheet material 1. The deviations labeled with reference numbers 4 and 5 are position deviations, conversely in the representation as shown in FIG. 2 angle deviations of the printed image 2 are shown with reference to the position on the printed sheet of sheet material 1.

[0025] In FIG. 2 the developing angular errors are labeled with reference number 6. The printed image 2 can be printed in the indicated positions onto the surface of the printed sheet material 1, this material being conveyed, viewed in the conveyor direction 22, with its front edge 23 forward.

[0026]FIG. 3 shows in a schematic view the turning register, and the offsets which develop between the printed images 2 on the front and back of the sheet material 1 can be characterized with reference number 7. These offsets are labeled with reference number 7 or Ax and Ay in FIG. 3. The turning register plays a part especially in translucent types of paper and when printing booklets.

[0027]FIG. 4 shows in a schematic side view the interface of sheet alignment and feed onto a transport belt. An alignment unit 8 is connected upstream of a transport belt 10 which runs around a feed roller 11 and a control roller 12; on the surface of the belt the sheet material 1 is held in the conveyor plane 9. After passing the alignment unit 8 which will be described in greater detail below, the aligned sheet material 1 on the surface of the transport belt 10 travels to the conveyor plane 9. After passing the feed roller 11 the sheet material 1 is captured by an adjustment flap or adjustment lip which can be moved in the adjustment direction 13. The adjustment lip or adjustment flap can be a plastic component which can be moved from the adjusted position 13.1 in the stopped position 13.2; this is shown here only schematically in solid or broken lines. The adjustment flap or adjustment lip presses the sheet material 1 onto the surface of the transport belt 10 in the aligned state of the sheet material 1. After passing the pressure element 13 the sheet material 1 which is held on the surface of the transport belt 10 passes a charging unit 14. In this charging unit 14, inside a hood-shaped cover there is an electrode 15 which provides for static charging of the sheet material 1 and thus for its adhesion to the surface of the transport belt 10.

[0028] A front edge sensor 17 follows the charging unit 14 which is shown only schematically in FIG. 4. This sensor consists of a radiation source 18 which is located underneath the sheet conveyor plane 9 and to which a lens arrangement 19 is series connected. The radiation field 20 proceeding from the lens arrangement 19 penetrates the conveyor plane 9 and is incident on a diaphragm arrangement which is located above the conveyor plane 9 of the sheet material 1. The diaphragm arrangement precedes a receiver 21 which senses the presence of the front edge 23 of the sheet material 1.

[0029]FIG. 7 shows in a plan view the alignment unit 8 with its components which are shown schematically here. The alignment unit 8 is reached by the sheet material 1 which is conveyed in the conveyor direction 22. The front edge 23 of the sheet material 1 is offset with respect to the conveyor direction 22, by which also the side edge 24 of the sheet material 1 begins to run skewed. As soon as the front edge 23 of the sheet which is in the skewed position with respect to the conveyor direction 22 runs over a first photoelectric barrier 26, the drives 27, labeled M 1 and M 2, which drive rotation elements 25 via individual axles 32, are accelerated to the feed rate. Triggering of the drives 27 (M 1 or M 2) which is initiated via the photoelectric barrier 26 ensures that each copy of the sheet material 1 comes into contact with identical peripheral segments of the rotation elements 25 which can be made as segmented rollers in the preferred embodiment. Any developing differences in the feed motions which can be attributed to the dimensional and shape tolerances of the two rotation elements 25 thus occur in the same way for each copy of the sheet material 1 and can be easily calibrated out.

[0030] After the two rotation elements 25 are set into rotation by passing the first photoelectric barrier 26, the sheet material 1 is transported with the feed rate over another sensor unit 30.1 which follows the first photoelectric barrier 26. As soon as the first of the two sensors of the sensor pair 30.1 has detected the front edge 23 of the sheet material 1, a counter unit begins to count in motor steps. The counting process is then ended and the ascertained difference is retained when the second sensor of the sensor pair 30.1 operates. The counter state which has been determined in this way allows determination of a correction value which is relayed as additional feed to the segmented roller drive 27 which was started last, i.e. either the drive 27 which is labeled M 1, or the drive 27 which is labeled M 2. In this way the corresponding body of revolution 25 which is made as a segmented roller is accelerated to an increased feed rate until the stipulated path difference is completely equalized. At the end of this correction process the front edge 23 is oriented exactly perpendicularly to the conveyor direction 22. After completed correction of the position the sheet material 1 in the conveyor direction 22 is continuously transferred from the first pair of segmented rollers 25 to the other pair of segmented rollers 25 which follow them and which can be accommodated on a common axis 31. At this point the segmented roller pair 25 which is driven via the drive 27 or M 1 and the drive 27 (M 2) is turned off and moves into a neutral position.

[0031] The sheet material 1 which is now correctly aligned with respect to its angular position now runs onto a sensor array 30 in which the position of the side edges 24 of the sheet material 1 is measured. The change in position for the drive 27 which is labeled M 4 and which has a drive shaft which extends parallel to the conveyor direction 22 of the sheet material 1 is determined from the established measured value. By means of this drive 27 which is held in a second orientation 29 the position of the sheet material 1 parallel to the direction 22 in which it is running is corrected (compare FIG. 7).

[0032] Afterwards, the sheet material 1 which is aligned in its angular position and its lateral position in this way runs underneath an adjustment element, in the form of an adjustment flap, or an adjustment lip, which has been placed in a position 13.1 or 13.2, onto the transport belt 10 in order to run into the downstream printing unit in the correctly aligned position. FIG. 6 shows one embodiment of the rotation elements 25 which are located above the conveyor plane 9 and which are held in the alignment unit 8. The rotation elements 25 in one preferred embodiment can be made as segmented rollers which have a peripheral surface 33 which is characterized by an interruption. The segmented rollers 25 rotate in direction 34, characterized by the illustrated arrow, and describe roughly a ¾ circle with reference to their axis of rotation. Underneath the respective segmented roller 25 the roller which supports the sheet material 1 is shown. This support roller can be made either in one piece or can consist of a roller core with a coating held on it. The bodies of revolution which are made as the segmented rollers 25 are shown in the neutral position in the left-hand part of FIG. 6, while in the right-hand part of FIG. 6 they grip one copy which is conveyed in the conveyor direction 22 of the sheet material 1 by its peripheral surface 33 and transport it according to the direction of rotation 34 in the conveyor direction 22. FIG. 4 shows the correction of the angular position of the sheet material 1 as it passes the alignment unit 8. In the position of the sheet material 1 shown in FIG. 7 its front edge 23 has just reached the last sensor of the sensor pair 30.1, so that now the drive 27 of the segmented roller 25, which drive is labeled M 1, can be activated to equalize the angular position of the sheet material 1 with reference to its conveyor direction 22. It should be mentioned that in contrast to drives M 3 and M 4 which are joined to one another via a continuous drive shaft 31, the segmented rollers 25 which are connected to the drives M 1 and M 2, are each driven via individual shafts 22. After correction of the angular position of the sheet material 1 by activations of the respective drives 27 (M 1 and M 2) of the segmented rollers 25 at different speeds, the sheet material 1 undergoes correction of its side position. After measurement of the position of the side edges 30 of the sheet material 1 by the sensors 31, the sheet material 1 is now correctly aligned parallel to the conveyor direction 22 in which via the drive M 4 the displacement of the sheet material 1 takes place in its conveyor plane 9 before reaching the adjustment element 16 and before the sheet material 1 runs onto the surface of the transport belt 10. With drive 27 (M 3) oriented in the first orientation 28, via a common shaft 31, feed of the sheet material 1 with the front edge 23 correctly aligned is ensured, while it is aligned in its lateral position via the drive 27 (labeled M 4) which is held in the second orientation 29.

[0033]FIG. 8 shows the effect of tolerances, for example the cutting tolerances of the sheet material 1 to be processed, on the lateral alignment.

[0034]FIG. 8 shows two copies of the sheet material 1 on top of one another. Each copy of the sheet material 1 has a width 42 and a lengthwise extension viewed in the conveyor direction 22 which is labeled with reference number 43. Each copy of the sheet material 1 is transported with its front edge 23 forward in the conveyor direction 22, the front edge 23 adjoining the side edge 24 which runs essentially parallel. The offset 39 which in the copies of the sheet material 1 to one another [sic] and which follows from FIG. 8 causes activation of the lateral alignment elements at different times as a result of the different operating points 40 and 41 which arise. The two operating points 40 and 41 for the copies of the sheet material 1 which are shown on top of one another cause a different lateral alignment accuracy depending on the side register error 39 which occurs each time and which can fluctuate greatly depending on the print material and is the cause of an alignment result which is subject to defects. In this alignment process the cutting tolerances or the side register errors 39 of the sheet material 1, i.e. the right angle between the front edge 23 of the sheet and the adjoining side edges 24, are included in the alignment result. In printing pamphlets in which for example sheet material 1 is printed in A-3 format, accordingly is aligned with respect to the front edge 23 and side edge 24, then is folded to A 4 and stitched, and in which two pictures 37 are extended over two sides of two different sheets, the problem arises that the register accuracy on the folded spine can be achieved only within the framework of the cutting tolerance. The greater the cutting tolerances 39 were initially on the sheet material 1, the more inaccurately the register can be held on the folded spine, for example, in printing booklets.

[0035] The different operating points 40 and 41 which develop as a result of tolerances on sheet material 1 for the alignment elements 25 can be eliminated as shown in FIG. 9 by the position of the sheet material 1 in the conveyor plane 9 always being measured at the same position 35 with respect to a reference edge 23. This is done for example by the sensor elements which are embedded into the conveyor plane 9 and which can preferably be made as line sensors, for example as CCD lines, and as a diode line. These line sensors which are provided with the corresponding resolution allow controlled correction of the position of the sheet material 1 viewed in the conveyor direction 22 after recording the sheet position with respect to its reference edge 23. The position 37 of the printed images which are located on the sheet material 1 is defined with respect to the reference edge 23 by the distance 36 while it is defined with respect to the measurement point 35 on the side edge via the distance 38. Thus, in the finishing of the sheet material 1 the register accuracy is no longer limited by the cutting tolerances of the sheet material, so that register inaccuracies in booklet printing can no longer occur for images of two different sheets extending over two pages.

[0036] Reference number list

[0037]1 sheet material

[0038]2 printed image

[0039]3 frame

[0040]4 position error, Y direction

[0041]5 position error, X direction

[0042]6 twist error

[0043]7 offset, front and back

[0044]8 alignment unit

[0045]9 conveyor plane

[0046]10 transport belt

[0047]11 feed roller

[0048]12 control roller

[0049]13 adjustment element

[0050]13.1 first position

[0051]13.2 second position

[0052]14 charging unit

[0053]15 electrode

[0054]16 support

[0055]17 front edge sensor

[0056]18 radiation source

[0057]19 lens

[0058]20 radiation field

[0059]21 radiation receiver

[0060]22 conveyor direction

[0061]23 front edge (reference edge)

[0062]24 side edge

[0063]25 segmented roller

[0064]26 photoelectric barrier

[0065]27 drive, segmented roller

[0066]28 first orientation, drives

[0067]29 second orientation, drives

[0068]30 line sensor

[0069]30.1 sensor pair

[0070]31 common shaft

[0071]32 individual shaft

[0072]33 periphery of the segmented roller

[0073]34 direction of rotation

[0074]35 reference point, side edge

[0075]36 reference distance x

[0076]37 position of the printed image

[0077]38 reference distance y

[0078]39 error (tolerance)

[0079]40 operating point 1

[0080]41 operating point 2

[0081]42 width

[0082]43 lengthwise extension

[0083] 

1. Process for alignment of sheet material (1) which is processed in a machine which processes sheet material (1), and is aligned thereto in one conveyor plane (9) laterally and with respect to its twisting (6) with reference to its conveyor direction (22), wherein the position of the sheet material (1) in the conveyor plane (9) is always acquired on each copy at a point (35) on one side edge (24) of the sheet material (1) with a position which is defined to the reference edge (23) of the sheet material (1).
 2. Process as claimed in claim 1, wherein a line sensor (30) is used to determine the position of the sheet material (1).
 3. Process as claimed in claim 2, wherein CCD lines (30) are used to determine the position of the sheet material (1).
 4. Process as claimed in claim 2, wherein diode lines are used to determine the position of the sheet material (1).
 5. Process as claimed in claim 1, wherein the sheet material (1) is further conveyed during its alignment in the conveyor direction (22).
 6. Process as claimed in claim 1, wherein the formats of the sheet material (1) which are to be variably processed and its cutting tolerances (39) do not influence the image position (37) by measuring the position of the sheet material (1) always at the same locations (35, 23).
 7. Process as claimed in claim 1, wherein the position of the operating points (40,41) which is dependent on the original error (39) of the sheet material (1) for activation of the lateral alignment elements (25, 27) is replaced by defined acquisition of the position (35, 23) of the sheet material (1).
 8. Process as claimed in claim 7, wherein the position 37 of the printed image is defined relative to the position of the sheet material (1) via reference distances (36, 38).
 9. Process as claimed in claim 8, wherein the first reference distance is formed by the distance of the printed image (37) from the reference edge (23) of the sheet material (1).
 10. Process as claimed in claim 8, wherein the second reference distance is formed by the distance of the printed image (37) from the reference point (35) of the side edge (24) of the sheet material (1)>. 